Study yields mixed findings about microbes on organic produce

May 19, 2004 (CIDRAP News)  A recent comparative study of organic and conventionally grown produce on farms in Minnesota showed that the organic produce was virtually free of pathogenic bacteria but was more likely to have fecal contamination from manure used as fertilizer.

The study also showed that produce from certified organic farms was less likely to have fecal contamination, as represented by nonpathogenic Escherichia coli, than produce from uncertified farms. Certified organic farms are required to follow federal guidelines designed to minimize the risk of pathogens in manure used as fertilizer, according to the study's senior author, Francisco Diez-Gonzalez, PhD, of the University of Minnesota in St. Paul.

Diez-Gonzalez said the findings contradict the impression from media reports that organic produce is more likely to cause illness than conventional produce is. The study was published this month in the Journal of Food Protection.

"The media have portrayed that organic vegetables have a lot of foodborne pathogens. Our data doesn't support that," he told CIDRAP News. "But it does seem to confirm the belief that it [organic produce] is more susceptible to fecal contamination. The good news is that if you are certified, your chance of fecal contamination decreases significantly." Diez-Gonzalez is an assistant professor of food science and nutrition in the university's College of Agriculture, Food and Environmental Sciences and the College of Human Ecology.

Previous studies comparing organic and conventional produce have focused on produce in stores rather than on farms, whereas Diez-Gonzalez and three colleagues collected produce from the fields. "The reason we did this was to try to answer the question whether the organic practices at the farm had a great impact on the prevalence of the microorganisms," he said.

Farmers were recruited for the study at workshops and through personal visits and phone contacts. The researchers collected 476 produce samples from 32 organic farms and 129 samples from eight conventional farms, all in central and southern Minnesota. Eight of the organic farms were certified by accredited agencies; the rest were not certified but reported using organic practices. All of the organic farmers reported using manure as their main fertilizer, and four of the conventional farmers used manure in addition to chemical fertilizer.

Produce samples that were analyzed included tomatoes, leafy greens, lettuce, green peppers, cabbage, cucumbers, broccoli, strawberries, apples, and several other items. The samples were not washed before being analyzed.

Ordinary E coli was found in 9.7% of organic produce samples, versus 1.6% of the conventional produce, a significant difference, according to the report. However, E coli prevalence in produce from certified organic farms was 4.3%, which was not significantly higher than the level in conventional produce. The E coli prevalence in produce from uncertified organic farms was 11.4%, significantly higher than in the certified organic produce. In addition, 59% of uncertified organic farms had at least one sample with E coli contamination, versus only 12% of certified organic farms.

"Ours is the first study that suggests a potential association between organic certification and reduced E. coli prevalence," the report says. "Further research is recommended to confirm this finding."

E coli was 19 times more prevalent in produce from organic farms that used manure or compost less than a year old as fertilizer, compared with organic farms that used older materials, according to the report. In addition, E coli was found 2.4 times more often on produce from organic farms that used cattle manure as compared with farms using other kinds of manure. Among types of produce, organic lettuce had the highest E coli contamination, at 22% of samples (12 of 49).

Diez-Gonzalez said agencies that certify organic farms require them to follow US Department of Agriculture organic farming guidelines designed to eliminate pathogens in manure used as fertilizer. The guidelines deal with minimum temperature for composted manure and the minimum time interval between manure application and harvest. Manure that is not composted may not be used less than 120 days before harvest for crops in close contact with the soil and and 90 days before harvest for other crops, he said. However, he said the guidelines don't say anything about how old manure should be before it is used.

No E coli O157:H7 or other pathogenic E coli strains were found in any of the produce samples, the report says. But Salmonella was found in one organic lettuce sample and one organic green pepper from separate farms. "Based on the absence of E coli O157:H7 and the very low Salmonella prevalence, the assertion that organic produce has greater pathogen contamination does not seem to be supported," the article says.

Diez-Gonzalez said the current findings were from the first year of a 3-year study that began 2 years ago. The second year of the study yielded no findings of E coli O157:H7 or Salmonella on organic produce samples, though those findings have not yet been published, he said.

Craig Hedberg, PhD, a foodborne disease expert and associate professor of environmental and occupational health at the University of Minnesota in Minneapolis, said a limitation of the study is that it dealt only with Minnesota, which is not a major producer of fruits and vegetables. "It may be relevant to a broader question, but the results aren't all that generalizable to the world at large," he said.

However, Hedberg said the finding that the use of manure less than a year old was linked with more E coli contamination on produce is probably important. "Organic farmers might want to look at this and say, 'I have to look at how I manage my manure before I use it,'" he said.